1. Field of the Invention
Embodiments disclosed herein generally relate to a downhole isolation tool. More specifically, embodiments disclosed herein relate to a downhole isolation tool having a ball seat mandrel having two or more ball seats. Additionally, embodiments disclosed herein relate to a downhole isolation system having two or more downhole isolation tools. Further, embodiments disclosed herein relate to methods of running a downhole isolation system into a well and isolating zones of a well with a downhole isolation system.
2. Background Art
In drilling, completing, or reworking wells, it often becomes necessary to isolate particular zones within the well. In some applications, downhole isolation tools are lowered into a well to isolate a portion of the well from another portion. The downhole tool typically includes a sleeve coupled to a ball seat. A ball may be dropped from the surface and seated in the ball seat to seal or isolate a portion of the well below the tool from a portion of the well above the tool. More than one downhole isolation tool may be run into the well, such that multiple zones of the well are isolated.
The downhole isolation tool may be run in conjunction with other downhole tools, including, for example, packers, frac (or fracturing) plugs, bridge plugs, etc. The downhole isolation tool and other downhole tools may be removed by drilling through the tool and circulating fluid to the surface to remove the drilled debris.
The downhole isolation tool may be set by wireline, coil tubing, or a conventional drill string. The tool may be run in open holes, cased holes, or other downhole completion systems. The ball seat disposed in the downhole isolation tool is configured to receive a ball to isolate zones of a wellbore and allow production of fluids from zones below the downhole isolation tool. The ball is seated in the seat when a pressure differential is applied across the seat from above. For example, as fluids are pumped from the surface downhole into a formation to fracture the formation, the ball is seated in a ball seat to maintain the fluid, and therefore, provide fracturing of the formation in the zone above the downhole isolation tool. In other words, the seated ball may prevent fluid from flowing into the zone isolated below the downhole isolation tool. Fracturing of the formation allows enhanced flow of formation fluids into the wellbore. The ball may be dropped from the surface or may be disposed inside the downhole isolation tool and run downhole within the tool.
At high temperatures and pressures, i.e., above approximately 300° F. and above approximately 10,000 psi, the commonly available materials for downhole balls may not be reliable. Furthermore, as shown in FIGS. 1A and 1B, a conventional ball seat 36 includes a tapered or funnel seating surface 40. The ball 38 makes contact with the seating surface 40 and forms an initial seal. Based on the geometries of the seating surface 40 and ball 38, there is a large radial distance between the inside diameter of the seating surface 40 and the outside diameter of the ball 38. Thus, the bearing area between the seating surface 40 and the ball 38 is small. As the ball 38 is loaded to successively higher loads, the ball 38 may be subjected to high compressive loads that exceed its material property limits, thereby causing the ball 38 to fail. Even if the ball 38 deforms, the ball 38 cannot deform enough to contact the tapered seating surface 40, and therefore the bearing surface 40 of the ball seat 36 for the ball 38 remains small. An increase in ambient temperature can also increase the likelihood of extruding the ball 38 through the seat 36 due to decreased properties of the material. The mechanical properties of the ball 38 material may decrease, e.g., compressive stress limits and elasticity, which can lead to an increased likelihood of the ball cracking or extruding through the ball seat 36. Thus, in high temperature and high pressure environments, conventional downhole isolation tool, i.e., balls 38 and ball seats 36 within the downhole isolation tool, may leak or fail.
In open hole fracturing systems that use such balls and ball drop devices as means to isolate distinct zones for hydraulic fracturing treatment, different sized balls are used for each isolation zone. Specifically, in a wellbore where multiple zones are isolated, a series of balls are used to isolate each zone. A ball of a first size seals a first seat in a first zone and a ball of a second size seals a second seat in a second zone. The lowermost zone uses the smallest ball of the series of balls and the uppermost zone uses the largest ball of the series of balls. The smallest sized ball is typically ¾ inch to 1 inch in diameter. The corresponding ball seat and corresponding throughbore must have a diameter smaller than the ball to receive and support the ball. Typical hydraulic fracturing fluid rates are between 20 BPM (barrels per minute) and 40 BPM. The pressure drop through a restriction, i.e., the ball seat and corresponding axial throughbore, as small as ¾ inch is substantial. Such a pressure drop increases the total pump horsepower needed on location to complete an isolation job.
Accordingly, there exists a need for a downhole isolation tool that effectively seals or isolates the zones above and below the plug in high temperature and high pressure environments and provides sufficient through flow through the system.